Southern California Edison wants most of its testimony kept secret in a state investigation into what went wrong with the San Onofre nuclear plant.

In calling for the secret testimony, Edison and SDG&E said they may be required to submit confidential and proprietary information for the state investigation. And some of that information, they said in filings with regulators, might make public competitive details or may violate contracts they have with third parties like Mitsubishi.

Mitsubishi manufactured the faulty steam generators at the center of the state's inquiry.

John Geesman, an attorney for the Alliance for Nuclear Responsibility, has objected to Edison's request. He said if the California Public Utilities Commission agrees with Edison, public interest groups as parties to the state's investigation will get to see the companies' testimony only if they sign a nondisclosure agreement.

"But we would not be allowed to speak to the press or anyone else who had not signed such a nondisclosure agreement and you as a representative of the media would not have any access to the information," Geesman said.

Though it's been shut down since last January, customers spent more than $1 billion for San Onofre in the past year. Part of the state's inquiry will the settle the question of whether ratepayers should continue covering that bill. State regulators did not respond to requests for comment.